Two studies are being conducted which investigate attentional factors in auditory signal detection. The studies are being conducted simultaneously and are based upon the results of earlier research conducted in our laboratory. One study investigated the significance of stimulus change (a decrease or redistribution of masker energy or power, as opposed to the addition of a masker) on the processing of auditory signals embedded in a masking noise. This first study investigates the nature of the temporal, frequency, and channel aspects of such stimulus change effects. The second study investigates the ability of observers to process simultaneously or sequentially presented, independent stimulus information in two or more conceptually different input channels. The nature and significance of any limits on the ability of the observers to time-share is being investigated. These studies are intended to help delineate and significantly clarify the role of attention in the processing of auditory stimulus information. BIBLIOGRAPHIC REFERENCES: Ahroon, W.A., and Pastore, R.E. Selective Attention II. Channel separation in two-channel frequency discriminations. Journal of the Acoustical Society of America, 1977. Ahroon, W.A., Pastore, R.E., and Wolz, J.P. Selective Attention I. Two-channel simultaneous frequency difference limen. Journal of the Acoustical Society of America, 1977, 61, in press.